The present invention relates to video signal processing, and more particularly to a method of concealing temporal shift when using a digital effects system and a video switcher.
Television production switcher-based effects, such as mixes, wipes and keys where the pixels maintain their positions within the video raster, have a processing time to accomplish these effects that is typically shorter than two video scan lines. For transform effects, such as slides and pushes, pixels are moved to different locations in the raster and these effects are performed by a digital effects system. Digital effects systems have a latency--they delay the picture by a video frame or longer. This is necessary because pixels near the bottom of the picture are scanned after those nearer the top, and no pixel may be advanced up the raster before it is scanned into the system. Therefore in digital effects systems the picture is stored in a video frame memory and then read out from that memory using a different order of memory addresses, but necessarily delayed by at least one frame.
A frame memory is not necessary for performing video switcher effects and historically it has been omitted from the switcher's signal path because of its expense and complexity. In systems having both a video switcher and a digital video effects system, the digital video effects system is usually switched in and out of the switcher's processing path as an allocatable resource because of its expense and the desire to insert at many possible points along the path. When the digital video effects system is switched in, it adds its latency delay. At the moment of the switch there is a temporal "hop" in the resultant picture. The extent to which the temporal hop becomes noticeable and undesirable depends upon the picture content and the conditions when the switch is made.
Human perception is such that the extra delay through the digital video effects system is undetectable. However a change in the delay is quite noticeable if there is motion in the picture to provide a frame of reference. With video having moving objects a temporal hop becomes an easily seen spatial hop. Some things may be done to mask the temporal hop. If the scenes before and after the switch are the same and static, no spatial hop occurs and the switch is invisible. The temporal hop also may be masked in effects where the transformed image slides over the previous background image and the system cuts immediately to the non-transformed equivalent of the new image. In this case the spatial hop is masked by the transform's motion. A third way to smooth over the temporal hop is to do a video mix between the undelayed signal and its frame delayed counterpart. By doing so an abrupt movement is blurred and somewhat hidden.
Switcher operators may perform a mix as described above to mitigate the temporal hop. However to do so on an existing switcher required advance planning and a multi-step operation of mixing, performing the transform and then possibly mixing a second time to release the digital video effects system. These steps are at odds with the quick pace of a live video operation. Operators prefer to select a new scene and make the transition in one simple motion.
What is desired is a "hop hider" that automatically conceals the temporal shift when using a digital video effects system with a video switcher.